The present invention relates to a document transport system and, more particularly, to a document transport system with a document platen adapted to minimize drag forces associated with the movement of documents across its surface.
As demands for faster copying and printing machines have intensified, increasing attention has been given to improving the operation of the mechanisms used to feed original documents into an exposure area. The original documents are fed, either automatically (RDH systems) or semi-automatically (SADH systems) at speeds up to 35 inches/second onto a transparent copying window (platen). Whatever the transport system used, it must be capable of moving the documents from some initial position onto the platen, aligning and/or maintaining the document in the proper registration during exposure, and subsequently removing the document from the platen, without interfering with the transparency of the platen or the exposure of the document therethrough.
Various mechanisms are known in the art to transport the document into, and out of, a platen exposure area. U.S. Pat. Nos. 3,889,943 and 4,231,561 disclose arrangements wherein the documents are moved across the platen surface by flexible rotating belts overlying the platen. In U.S. Pat. Nos. 4,335,954, 4,213,603 and 4,171,128, for example, rollers are used to accomplish the same purpose. A problem with transport systems of the type disclosed in these references is that electrostatic forces associated with the movement of the paper across the platen are introduced. These forces create a "drag" on the transported document, resulting in misfeeding and especially misregistration problems. One solution to this problem is disclosed in Japanese laid-open publication No. 55-65637. As disclosed in this publication, documents to be copied are transported to the top of a platen by a conveyor belt mechanism. The surface of the glass platen is coated with an optically transmissive conductive material grounded through a series of rollers. This arrangement acts to dissipate the static charges which accumulate during frictional transport of the document across the platen. While this arrangement may be suitable for the document size conveyor belt used in the disclosed system, it has been found that systems using more localized document transport systems; e.g. rollers, experience a continued electrostatic drag force in those areas underlying the localized transprent mechanism. The present invention is therefore directed to an improved transport system which includes a glass platen selectively modified to incorporate anti-static and improved optical properties. More particularly, the invention is directed to a document transport system wherein localized document transport means are positioned in operative frictional engagement with a document so as to move said document into a registration position prior to exposure and to remove said documents from said platen following exposure, an improved platen comprising a glass base coated with at least a layer of grounded, optically transmissive, conductive material, said conductive material being omitted in an area generally underlying the frictional contact area of said localized transport means.